Import exam grades: ignore no score rows

[annagav]

What are the relevant tickets?

Fix #4756

What's this PR do?

Update the management command to ignore records that do not have a score.

How should this be manually tested?

Run the management command:
./manage.py import_edx_exam_grades <file>

[codecov-io]

Codecov Report

Merging #4759 (36a42bf) into master (4b4830a) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4759      +/-   ##
==========================================
- Coverage   94.41%   94.39%   -0.03%     
==========================================
  Files         512      512              
  Lines       23344    23349       +5     
  Branches      967      967              
==========================================
  Hits        22040    22040              
- Misses       1201     1206       +5     
  Partials      103      103              

Impacted Files	Coverage Δ
...xams/management/commands/import_edx_exam_grades.py	0.00% <0.00%> (ø)

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[arslanashraf7]

Looks good 👍 , Just a small change suggested. Feel free to merge on your ease.

FYI, Since i didn't have the sample file, i was able to test it after generating my own csv file with required columns.

[arslanashraf7]

Suggested change

	'percentage_grade': score / 100.0 if row['score'] else 0,
	'percentage_grade': score / 100.0 if score else 0,